Heretofore, an HSDPA (High Speed Downlink Packet Access) mobile communication system has been known as a mobile communication system in which a retransmission control is performed (see Non-Patent Document 1, for example).
With reference to FIG. 1 to FIG. 7, a description will be given of the retransmission control in the conventional HSDPA mobile communication system.
The retransmission control in the HSDPA mobile communication system having the configuration shown in FIG. 1 is implemented by an “HARQ (Hybrid Automatic Repeat reQuest) retransmission control” in an MAC-hs layer and an “ARQ retransmission control” in an RLC layer.
In this HSDPA mobile communication system, the ARQ retransmission control in the RLC layer is configured to ultimately compensate for a packet reception error and a packet decoding error which cannot be compensated in the MAC-hs layer. Moreover, in the HSDPA mobile communication system, the ARQ retransmission control in the RLC layer is configured to ultimately compensate for missing (loss) of a packet due to, for example, a false detection of transmission acknowledgement information (ACK/NACK) in the HARQ retransmission control (for example, a mobile station falsely detects an ACK as being an NACK, or a mobile station falsely detects reception of an ACK from a mobile station actually in a DTX state).
With reference to FIG. 2A to FIG. 2F, a specific description will be given of the retransmission control in the HSDPA mobile communication system. Hereinbelow, the description will be given under the assumption that a single HARQ process is performed. In addition, a “TSN (Transmission Sequence Number)” assigned to each packet cyclically takes a value from 0 to 15.
In FIG. 2A, an MAC-hs layer of a receiving side apparatus (mobile station) 30 successfully receives and decodes a packet “TSN=15”. Accordingly, the MAC-hs layer stores the packet “TSN=15” in a sequence control buffer 35, and concurrently extracts a packet “TSN=10” from the sequence control buffer 35 and then the MAC-hs layer forwards the extracted packet to an RLC layer. Then, the “TSN” of the packet which the receiving side apparatus 30 expects to receive next is set at “0”.
In FIG. 2B, however, the packet “TSN=0” is lost in the MAC-hs layer of the receiving side apparatus 30, because of any of the above-mentioned errors (such as reception error, decoding error, and false detection). Accordingly, although the “TSN” of the packet which the receiving side apparatus 30 expects to receive next remains at “0”, a transmitting side apparatus (base station) 10 transmits a packet “TSN=1” to the receiving side apparatus 30 since the transmitting side apparatus 10 has not received an NACK for the packet “TSN=0”.
Thereafter, the MAC-hs layer of the receiving side apparatus 30 successfully receives and decodes the packet “TSN=1”. Accordingly, the MAC-hs layer stores the packet “TSN=1” in the sequence control buffer 35, and concurrently extracts a packet “TSN=12” from the sequence control buffer 35 and then forwards the extracted packet to the RLC layer.
Then, the same operation is repeated in FIG. 2C to FIG. 2E. Meanwhile, the “TSN” of the packet which the receiving side apparatus 30 expects to receive next remains at “0”.
Thereafter, in FIG. 2F, the MAC-hs layer of the receiving side apparatus 30 successfully receives and decodes a packet “TSN=5”. Accordingly, the MAC-hs layer stores the packet “TSN=5” in the sequence control buffer 35.
At this time, the MAC-hs layer of the receiving side apparatus 30 attempts to extract the packet “TSN=0” from the sequence control buffer 35 so as to pass the packet “TSN=0” to the RLC layer, but fails to extract the packet “TSN=0” since the packet “TSN=0” is not stored in the sequence control buffer 35.
Consequently, an ARQ entity 36 of the RLC layer detects the missing (loss) of the packet “TSN=0”, and requests the transmitting side apparatus 10 to retransmit the packet “TSN=0” by the ARQ retransmission control.
As shown in FIG. 1, in the HSDPA mobile communication system, the base station is configured to transmit L1/L2 control information shown in FIG. 3 to the mobile station through an “HS-SCCH (High Speed Shared Control Channel)” as a shared physical control channel for transmitting the L1/L2 control information, and to transmit a packet to the mobile station through an “HS-PDSCH (High Speed Physical Downlink Shared Channel)” as a shared physical data channel.
Here, as shown in FIG. 3, the HS-SCCH is associated with the HS-PDSCH. The mobile station is configured to receive a packet included in the HS-PDSCH associated with the received HS-SCCH, on the basis of the L1/L2 control information included in the HS-SCCH.
Moreover, in the HSDPA mobile communication system, an “HS-DSCH (High Speed Downlink Shared Channel)” is configured to be multiplexed on the HS-PDSCH, as a transport channel.
Moreover, in the HSDPA mobile communication system, a single protocol data unit (“MAC-hs PDU”) (hereinafter referred to as a packet) is configured to be transmitted at each TTI (Transmission Time Interval) of the HS-DSCH. Note that the “TSN” of each packet is included in the header portion of the packet.
Moreover, in the HSDPA mobile communication system, the mobile station is configured to determine whether or not each TTI of the HS-DSCH is assigned to the mobile station itself, on the basis of a “UE identity (16 bits)” included in the L1/L2 control information (see FIG. 4) received through the HS-SCCH.
Then, when the TTI of the HS-DSCH is assigned to the mobile station itself, the mobile station is configured to determine whether or not a packet to be transmitted at the TTI is a new packet or a retransmission packet, on the basis of an “NDI (New Data Indicator (1 bit))” in the L1/L2 control information (see FIG. 4). According to the determination result, the mobile station is configured to perform the HARQ retransmission control in the MAC-hs layer.
Here, the “NDI” used in the conventional HSDPA mobile communication system is configured of 1 bit, and is configured to be updated when a new packet is transmitted. Specifically, the “NDI” is configured to take two values alternately, for example, “0”→“1”.
Non-Patent Document 1: 3GPP T525.308